In that case, it does not work to use the definition that states 'half-life is the time required for exactly half of the entities to decay'. Note the consequence of the law of large numbers: with more atoms, the overall decay is more regular and more predictable.Ī half-life often describes the decay of discrete entities, such as radioactive atoms. The number at the top is how many half-lives have elapsed. Simulation of many identical atoms undergoing radioactive decay, starting with either 4 atoms per box (left) or 400 (right). The accompanying table shows the reduction of a quantity as a function of the number of half-lives elapsed. Half-life is constant over the lifetime of an exponentially decaying quantity, and it is a characteristic unit for the exponential decay equation. Rutherford applied the principle of a radioactive element's half-life in studies of age determination of rocks by measuring the decay period of radium to lead-206. The original term, half-life period, dating to Ernest Rutherford's discovery of the principle in 1907, was shortened to half-life in the early 1950s. The converse of half-life (in exponential growth) is doubling time. For example, the medical sciences refer to the biological half-life of drugs and other chemicals in the human body. The term is also used more generally to characterize any type of exponential (or, rarely, non-exponential) decay.

The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable atoms survive. Half-life (symbol t ½) is the time required for a quantity (of substance) to reduce to half of its initial value.

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